A slide fastener typically is formed by a pair of longitudinally extending and parallel textile tapes having confronting edges that carry interleavable coupling elements. These elements, which are typically made of a synthetic-resin coiled or meandered monofilament, do not extend the full length of the respective tapes and are joined together at their one ends by a so-called bottom end stop and each carry at their opposite ends a so-called top stop. A slide can move along both elements and is constructed such that on longitudinal movement from the bottom stop toward the top stops it interleaves, that is joins, the two coupling elements, and on opposite movement it separates them.
Such fasteners are typically made as described in commonly assigned U.S. Pat. No. 4,932,113 in a mass-production operation starting from a basic workpiece comprised of two very long parallel tapes whose confronting edges carry full-length coupling elements that are usually joined together. In a first machine a gap is formed in the joined coupling elements, same being cut away or otherwise removed for short distances at locations spaced along the tapes by the length of the fasteners to be made. Then the bottom end stops are fitted to the joined coupling elements at what is normally relative to the direction of travel of the tapes the trailing edge of each gap. A slider is then fitted to the elements from the leading edge of each gap, being slid on in a direction tending to separate the elements. Subsequently the top stops are applied to the separated coupling elements immediately downstream of the slider at the leading end of each gap. Finally the tapes are cut transversely across generally through the center of the gap, separating out the individual fasteners.
The gapping machine typically comprises a transport system that pulls the workpiece through a guide provided at its upstream end with a clamp and, immediately downstream therefrom, a punch and die system or the like that serves to cut out the coupling elements. As a rule the transport system is a pair of continuously driven pinch rollers frictionally engaging the workpiece with sufficient force to advance it through the machine, which offers substantial resistance to advance of the workpiece. The clamp is closed periodically to arrest the workpiece, whereupon the cutter itself works. During the time the clamp is closed the workpiece slips between the pinch rollers.
Such an arrangement has two main problems. First of all the spacing of the gaps is not very exact. The clamp is operated at a rate dependent of the peripheral speed of the transport rollers, so that it closes at exactly spaced intervals. Unfortunately the stop-and-go nature of the operation plus the varying friction between the workpiece on one side and the clamp and rollers on the other adds some range for error to the actual spacing of the gaps, as any slippage as the tape starts and/or stops is translated into a placement error.
Second, the transport rollers continue to rotate even when the workpiece is stationarily clamped upstream, so that considerable tension is created in the workpiece. Depending on the coefficients of friction of the rollers and workpiece, the longitudinal stretchability of the workpiece, and the force with which the rollers are urged together, this can translate into a tension that is so great that a gap is formed that, once the workpiece is released, shortens. Thus in general the known system is inexact, and this inexactitude is particularly troublesome in automatic systems that sense the gap itself to trigger other manufacturing steps, like installing the end stops and slider.
In the above-cited parent application we disclose a method of forming longitudinally spaced gaps in a workpiece formed of a pair of longitudinally extending parallel tapes having confronting edges provided with longitudinally extending and transversely couplable coupling elements. This method comprises the steps of guiding the workpiece longitudinally through a gapping station, looping the workpiece upstream of the station in a variable-size loop, continuously pinching the workpiece downstream of the station between a pair of transport rollers without the possibility of substantial slippage between the transport rollers and the workpiece, and periodically clamping and longitudinally arresting the workpiece at the station and cutting the coupling elements therefrom at the station to form a gap in the workpiece. In accordance with this earlier invention the displacement of the workpiece between the loop and the station is detected and measured after each clamping and cutting of the workpiece at the station and an output is generated corresponding to this displacement. This output is in turn compared with a set point corresponding to a desired slide-fastener length after each clamping and cutting of the workpiece at the station and the rollers are rotated to longitudinally advance the workpiece after each clamping and cutting of the workpiece only until the actual-value output matches the set point.
Thus according to our earlier invention the workpiece is positively fed through the gapping machine by the desired length in an arrangement where there is no slippage Instead of accommodating a continuously arriving workpiece by allowing slippage during the gapping operation at the transport system, the excess arriving workpiece is taken up at the looper. The result is extremely accurate gapping of the workpiece.